[Data] Add SchedulingHints sibling field on BlockEntry / wire envelope

[goutamvenkat-anyscale]

Adds an opt-in, producer-supplied forecast about what the next consumer of a block will need (memory today; cpu/gpu/locality/strategy in future additions on the same dataclass). Designed to be layered on top of the BlockEntry foundation without disturbing existing call sites.

Surface:

• New module ray.data._internal.scheduling_hints with:
  • SchedulingHints frozen dataclass (memory: Optional[int]; additive)
  • stage_scheduling_hints(hints) — producer helper; writes to TaskContext.next_block_scheduling_hints before each yield.
  • stage_memory_hint(memory) — single-axis convenience.
• BlockEntry.scheduling_hints: Optional[SchedulingHints] — sibling field to metadata, defaulting to None.
• BlockMetadataWithSchema.scheduling_hints — wire-envelope field that carries the hint from worker to driver alongside the per-block metadata. from_metadata / from_block thread it through.
• TaskContext.next_block_scheduling_hints + consume_next_block_scheduling_hints() — staging slot consumed and cleared by _map_task after each yield, so a stale value can't silently mis-tag later blocks.
• RefBundle.scheduling_hints accessor — parallel list to block_refs / metadata, returns the per-block forecasts.

Plumbing:

• _map_task reads the staged hints, attaches them to the BlockMetadataWithSchema it pickles per yield.
• PhysicalOperator driver-side bundle assembly lifts hints from BMWS into BlockEntry.scheduling_hints so consumers see them via the bundle's accessor.

This PR ships the infra only — no operator currently stages or reads hints. The Download operator wires producer (file-size totals → memory forecast) and consumer (bundle hint sum → per-task memory resource) in a follow-on PR.

[gemini-code-assist]

Code Review

This pull request introduces the infrastructure for pre-scheduling hints, allowing producers to stage prospective resource forecasts (such as memory requirements) for downstream consumer tasks. The changes include adding a new SchedulingHints dataclass, updating TaskContext to stage and consume these hints, carrying them on the wire envelope via BlockMetadataWithSchema and BlockEntry, and integrating them into the map operator's block yielding process. Comprehensive unit tests have also been added to verify the staging, consumption, and serialization of these hints. I have no feedback to provide.